In the dairying and other food stuff microbiology, medical microbiology as well as in general microbiology, determination of the number of microbes contained in a sample is generally carried out by means of dilution plate methods. Many of the so-called international comparative methods are of this type.
In dilution plate methods, a certain volume of a sample to be examined or of a dilution thereof is inoculated in a petri dish. E.g. 10 to 15 ml of a sterile culture medium capable of solidifying (agar) and having a temperature of 45.degree..+-.1.degree. C. is poured on the sample. The sample is immediately mixed with the medium, and the solidification is allowed to take place at room temperature on a horizontal surface. The sample and the medium can also be mixed before the plating. The solidified medium forms a layer of even thickness on the dish. The solidified plates are turned upside down and placed in an incubator (incubation). The incubation temperatures (e.g. 10.degree. C., 30.degree. C., 37.degree. C., 44.degree. C., 55.degree. C.) and the incubation time (e.g. 2, 3, 6 and 10 days) depend on the method applied, mainly on what microbe groups are to be examined. The dishes are in a horizontal position during the incubation.
After the incubation, colonies in the petri dish are counted. The amount of the inoculum in the sample and the dilution ratios being known, the number of microbes per one unit volume or one unit weight of the sample, usually microbes/ml or microbes/g, can be determined from the number of countable colonies.
The medium may contain as wide a diversity of nutrients as possible in order that as many microbe types of the sample as possible would have the required growth conditions (a so called total colony count). The nutrients of the medium may also be restricted or known growth inhibiting compounds may be added to the medium. Such media are referred to as selective media, and the object is to make a certain microbe group distinguishable.
The counting can be carried out ocularly (by means of known axialiary devices) or by means of various electrical counting devices (automatic colony counters).
A high colony concentration in the dish restricts the counting. It is customary to include in the counting only plates the colony quantity of which does not exceed a determined limit; for instance, with plates having a diameter of 9 cm, the upper limit is regarded to be 300 colonies. In order to be able to examine samples having a high microbe concentration, it is necessary to use dilution techniques. In practice, it is usually always necessary to use dilutions in the plating work. The preparation of successive dilutions covers 70 to 90 percent of the working time required for the whole plating stage (depending on the supposed need of dilution). Correspondingly, the material need is a multiple of the number of dilutions.
Prior dilution plate methods have thus a disadvantage that they require the preparation of large dilutions series and the cultivation of several dilutions. A great number of dishes is thereby required; further, a lot of incubation space is needed. In other words, the methods are complicated and time-consuming.